The Formation, Development, And Photosynthetic Capabilities Of Two Types Of Algal Propagative Cells

Algae are usually known as special primary photosynthetic organisms, because of their various reproductive modes (i.e. asexual reproduction, sexual reproduction and parthenogenesis etc.) and their distinctive light harvest complexes (i.e. the phycobilisome in red algae and fucoxanthin-chorophyll-protein complex in brown algae). Thus, the algae propagative cells are ideal material for the study on the development and concomitant photosynthesis properties. Meanwhile, algae also play great role in restoration the degenerated coastal environments, thus, comprehensive understanding of the reproductive and developmental characteristics of algae propagative cells will be significant in paving the way for the seedling. In this study, we investigated the variations in morphology and PSII photosynthetic capabilities during the early development of tetraspores of Gracilaria vermiculophylla (Gracilariales, Rhodophyta), and the oogenesis process of Sargassum horneri (Fucales, Heterokontophyta) and concomitant changes of PSII capabilities.In the tetraspores of G. vermiculophylla, we observed two types of division, cruciate and zonate, and both could develop into multicellular bodies (disks). During the first84hours, tetraspores divided several times, but the diameter of the disks changed very little; thereafter, the diameter increased significantly. Scanning electron microscopy observations and analysis of histological sections revealed that the natural shape of the disk remains tapered over time, and the erect frond grows from the central protrusion of the disk. Cultivation of tissue from excised disks demonstrated that the central protrusion of the disk is essential for initiation of the erect frond. Freshly released tetraspores retained limited PSII photosynthetic capabilities; when the tetraspores attached to a substrate, those capabilities increased significantly. Thus, we propose, attachment of tetraspores to a substrate significantly enhanced their PSII photosynthetic capabilities, and triggered further development. In the disk, the PSII activity of both marginal and central cells was similar, although some degree of morphological polarity was present; the PSII photosynthetic capabilities in young germling exhibited an apico-basal gradient. The central protrusion of the disk is the growth point, may have transfer of nutritive material with the marginal cells. Within the young germling, the hetero-distribution of PSII photosynthetic capabilities might be due to the differences in cell functions.S. horneri produced female gametes exclusively on reproductive branches called receptacles. Along the length of a receptacle, the degree of maturity of female gametes differed from the distal to the basal ends, and this gradation reflected the development process of female gametes. Over the surface of receptacles, special cavities called conceptacles developed via invagination of surface epidermis cells. Next, most of the liner cells differentiated into paraphyses, while a few of them differentiated into female gametes. The PSII photosynthetic capabilities of oogonia continuously weakened with increasing development. The Fv/Fm value decreased continuously from0.55±0.01for newly developed oogonia to0.2-0.3for mature oogonia. However, the diameter of the oogonia increased approximately20times from initiation to maturity. We propose that the transfer of nutrient material from tissues or cells with high PSII photosynthetic capabilities to the oogonia may exist, and these tissues or cells likely are the epidermis cells of receptacles.